Method and apparatus for improving dynamic response of golf club

ABSTRACT

An apparatus and method for improving the dynamic response of a golf club is disclosed. One embodiment is a shaft extension comprising a cylindrical member having a lower sleeve portion and an upper housing portion. The sleeve is sized to be inserted into the grip end of a golf club shaft with the upper housing extending from the end of the shaft. The housing has a hollow, open chamber in which a weighted insert is inserted and removably secured. Interchangeable inserts with varying masses are provided such that the dynamic response of the club may be tuned by adjusting the location of grip-end node of the fundamental modeshape of the club&#39;s response to a predetermined input.

BACKGROUND

[0001] The present invention provides a method and apparatus forimproving the dynamic response or feel of a golf club as it strikes agolf ball during play. The golf swing can be divided into six majorcomponents: initial alignment coupled with alignment stability; the backswing; the forward swing; ball impact; dynamic response of the club;swing follow through. These swing components are applicable both to fullswing clubs and putters.

[0002] Although there are many products and prior patents relating toadjusting the swing weight, feel, or balance of a golf club, few if anyof these devices are directed towards improving the dynamic response, orfeedback, of the club to the golfer at ball impact. Most prior artdevices are aimed more specifically at the static or quasi-static feelof the club in the golfer's hand at the initial alignment, or during theback and forward swings. Such devices usually focus on the feel of theclub itself, not the feel of the shot through the club. The importanceof impact and dynamic response to the golfer's game are oftenoverlooked.

[0003] Impact is momentary, but it is at and immediately following thiscritical moment that the golfer feels his shot through the dynamicresponse of the club. As many golfers will confess, after impact oneoften knows where the ball is heading without having to actually see itstrajectory. The golfer has only one tactile interface to the club, andthat is through his hands which grasp the club's shaft on the grip. Itis thus through the golfer's hands gripping the shaft that the dynamicresponse of the club to the golfer's stroke is communicated. Thisdynamic response is a result of the vibration characteristics of theclub, and the golfer often perceives it simply as feel. Thus it followsthat if the club's dynamic response can be increased in this specificgripping area, the golfer will have a better feel for his shot.

[0004] The dynamic response of the club may be quantified in terms offinite element analysis and empirical modal analysis. As used herein,the term “grip end” refers to the end of the shaft to which the grip isaffixed, and the term “head end” refers to the end of the shaft to whichthe club head is attached. Although some mathematical models of the golfclub treat the grip end as a fixed boundary, the golfer's hands arecoupled only viscously to the golf club. Thus the additional boundarystiffness at the grip-hand interface is negligible, and a fixed boundarycondition does not apply. On the head end of the club, however, the massof the club head relative to the shaft dominates the vibrationcharacteristic. As a result, for finite element analysis, the club isbest modeled by a beam with free-pinned boundary conditions. The pinnedend corresponds to the head end while the grip end of the clubrepresents the free end.

[0005] Mathematically, the impact of the club head against the ball ismost analogous to an impulse. In response to such an input excitation,the golf club exhibits a certain modeshape, which comprises thefundamental mode and harmonics of the fundamental extending into higherfrequencies. In any dynamic system, the lowest frequency mode, in thiscase the fundamental, has the greatest amplitude and thus exhibits thelargest displacement characteristics when responding to an inputexcitation such as the ball-head impact. Consequently, thelarge-displacement, low-frequency dynamic response of the fundamentalmode has the potential to provide maximum feedback to the golfer. Bydefinition, the fundamental mode has two nodes, one near each end of theclub, at which (again, by definition) the amplitude of the waveform iszero.

[0006] Finite element analysis of a pinned-free beam predicts, andempirical testing of actual golf clubs confirms, that the node of thefundamental mode near the grip end of the club (hereinafter the“grip-end node”) is located at a point that is approximately 26% of thelength of the club from the grip end. This location happens to fallwhere most golfers grip the club. As a result, the amplitude of thefundamental mode is at a minimum at the interface of the golfer and golfclub, and thus the golfer's ability to feel the dynamic response of theclub is muted.

[0007] The present invention provides a method and apparatus forimproving the dynamic response of the golf club by moving the grip-endnode of the fundamental mode further up the shaft towards the grip endand thereby increases the amplitude of the fundamental at the golfer'sinterface with the club. This action in turn enhances the feel of theclub to the golfer.

SUMMARY

[0008] One embodiment of the invention comprises a shaft extension forimproving the dynamic response of a golf club, with the upper or gripend of the club being hollow. The shaft extension includes a cylindricalmember comprising a lower sleeve sized for snug insertion in the gripend of the shaft and an upper housing of a diameter slightly larger thanthe inner diameter of said grip end, whereby the housing extends fromthe shaft when the sleeve is inserted into the shaft. The housing has aninterior chamber with an opening at the top of the housing. The shaftextension also includes an insert of predetermined weight that isinserted into and removably secured to the chamber. This may beaccomplished by various means, such as by threading the body of theinsert and inner wall of the chamber, or by securing the insert to thehousing with a fastener such as a screw. This two part arrangementallows the club to be selectively weighted above the shaft forselectively improving its dynamic response without changing the overalllength of the shaft and shaft extension in combination.

[0009] Another embodiment of the invention includes improving thedynamic response of a golf club by selectively adding weight to the gripend of the club until the grip-end node of the fundamental modeshape ofthe club moves from a first position to a second position closer to saidgrip end, which allows the golfer to feel through his hands a greaterresponse of the club through increased amplitude of the fundamental atthe hand-grip interface of the club.

DESCRIPTION OF DRAWINGS

[0010] These and other features, aspects, structures, advantages, andfunctions are shown or inherent in, and will become better understoodwith regard to, the following description and accompanied drawingswhere:

[0011]FIG. 1 is a perspective exploded view of an embodiment of theapparatus of the present invention;

[0012]FIG. 2 is a sectional view of the embodiment of FIG. 1 assembledand installed on a golf club shaft;

[0013]FIG. 3 is a sectional side view of the cylindrical member shown inFIG. 1;

[0014]FIG. 4 is a sectional side view of the weighted insert shown inFIG. 1;

[0015]FIG. 5 is a sectional side view showing variably weighted insertswhich may be used with the cylindrical member of FIG. 3; and

[0016] FIGS. 6A-B depict an alternative embodiment of the apparatus ofthe present invention.

[0017] FIGS. 7A-B are graphical illustrations of the perturbation of thelocation of the grip-end node of fundamental modeshape effected bypracticing the method of the present invention.

[0018] FIGS. 8A-B show test weights which may be used in practicing themethod of the present invention.

DETAILED DESCRIPTION

[0019] As shown in FIG. 1, one embodiment of shaft extension 100comprises an elongated cylindrical body 200 and a weighted insert 300removably secured to the body 300. In the embodiment shown, a fastenersuch as retaining screw 400 performs this function. The shaft extension100 is designed to be inserted into the grip end of a hollow golf clubshaft 500. The shaft 500 may be a shaft for a putter or a full swingclub, although the embodiment illustrated in FIGS. 6A-B, describedbelow, is preferred for the latter.

[0020] Referring to FIGS. 1 and 2, the cylindrical member 200 comprisesa lower sleeve 210 having an outer diameter sized to allow it to beinserted into and fit snugly within the end of shaft 500, and an upperhousing 220 with an outside diameter approximately the same as orslightly larger than the outside diameter of the shaft 500. Optionally,the sleeve may have a longitudinal bore 215, such that it is hollow. Thediameter of the bore 215 may be varied to adjust the overall weight ofmember 200. If a particularly heavy weight is desired or required, thesleeve may be solid.

[0021] The upper housing 220 has a longitudinal chamber 225 sized toaccommodate the weighted insert 300. The chamber 225 terminates in athreaded receptacle 226. In the embodiment shown, receptacle 226 is of amuch reduced diameter, as compared to the chamber 225, and is sized toaccept the threaded end of screw 400. Further, lower portion 224 of thechamber 225 may have a tapered shape of reducing diameter leading intothe threaded receptacle 226. This shape is advantageous in that iteffectively guides the screw 400 to the opening of the receptacle duringassembly. Note that the taper need not extend fully to the opening ofthe receptacle to achieve this effect.

[0022] The weighted insert 300 is shown in FIGS. 1 and 4. In thisembodiment, the insert 300 comprises a body 310, an upper flange 320,and a longitudinal bore 330. The body 310 is sized to fit within thehousing 220 of the cylindrical member 200, and the flange 320 is ofapproximately the same diameter as the outside diameter of the housing220, such that the flange acts as a stop when the body 310 is insertedinto the housing 220. The longitudinal bore 330 accommodates the barrel410 of screw 400 and includes an enlarged recess 335 to receive thescrew's cap 420.

[0023] This embodiment is installed onto a golf club, without a gripinstalled, as follows. The sleeve 210 of cylindrical member 200 isinserted into the end of a hollow shaft 500. A small shoulder 230 isformed at the junction of the sleeve 210 and the housing 220, and thisshoulder 230 thus acts as a stop as the member 200 is inserted into theshaft 500. Consequently, the housing 220 extends from the end of theshaft 500. Note the shaft 500 may be shortened by the length of housing200 prior to installation to maintain the same overall club lengthbefore and after installation, or if the shaft 500 may be trimmed lessthan the length of housing 200 or not at all if the golfer desires aslightly longer club. A suitable adhesive or epoxy may be applied to theouter surface of the sleeve 210 to affix it permanently within the shaft500. Further, the outer surface of sleeve 210 may be roughened orknurled to facilitate the fit and adhesion within the shaft. The insert300 is inserted into the housing 220 of the cylindrical member 200, andthe barrel 410 of the screw 400 is then inserted through the bore 330 inthe insert 300. The screw 400 is threaded into the recess 226, fixingthe insert in position. Optionally, the body 310 of insert 300 may be ofa slightly reduced diameter, such that it is not in contact with theinner wall of the housing 220 (i.e., there is a small air gap betweenthe two). Thus, the insert 300 simply drops into place with the flange320 bearing against the upper opening of the housing 220. Further, inthis case the cap 420 of screw 400 and the recess 335 of the bore 330may be cooperatively sized such that the cap 420 is actually press fitinto the recess 335 as the screw 400 is threaded into the receptacle 226during assembly. As a result, the insert 300 then turns with the screw400, which allows for easy removal and replacement of the insert 300.

[0024] The component parts of the shaft extension 100 may be constructedfrom any suitably durable and rigid material, including metals such asbrass, aluminum, lead, tungsten, titanium, stainless steel, nickel andtheir alloys. For simplicity, when a metal is identified herein, such astungsten, such identification refers to the metal and its alloys knownin the art. It is contemplated that composite materials also could beused. The component parts may be manufactured by any conventionalmachining, casting, molding, or other fabrication technology. Alloys ofbrass and aluminum are preferred for their relatively low cost,availability, durability, and ease with which they may be worked.Utilizing inserts of brass, aluminum, and tungsten also increases therange of the weight of the inserts due the different densities of themetals.

[0025] As shown in FIG. 5, a plurality of interchangeable weightedinserts 300 of varying sizes are provided to allow selective weightingof the cylindrical member 200, which results in a shaft extension 100 ofa precise and desired weight. As noted above, the inserts 300 may beconstructed of materials of different densities to allow a broad rangeof weights to be added to the club. For example, the inserts may rangefrom a small aluminum insert of 5 grams to a tungsten insert of 250grams or more that occupies the entire chamber 225 of housing 220.Likewise, the cylindrical member 200 may be constructed from arelatively heavy material such as brass or a relatively light materialsuch as aluminum as needed or desired for the particular application.Thus, the weight of the insert may be adjusted, without changing thelength of the shaft extension 100 or the combination of the extension100 and shaft 500. As described below, weights of varying mass areinterchanged to achieve the desired dynamic response in accordance withthe method of the present invention.

[0026] By way of example, one embodiment of the cylindrical member 200is 3.125 inches long, of which the upper housing 220 is 1.900 inches andthe lower sleeve 210 is 1.250 inches. The outer diameter of the upperhousing 220 is 0.600 inches, with the diameter of the chamber 222 being0.516 inches. The chamber 222 is 1.790 inches long, with the tapered end224 accounting for approximately 0.09 inches of this length. The chamber222 may be drilled with a standard 33/64 bit with a 118 degree point.The threaded receptacle 226 is approximately 0.34 inches long, with a10-24 thread, and is approximately 0.141 inches in diameter (9/64 drillsize or equivalent for 10-24 thread). The outer diameter of the lowersleeve 210 is 0.540 inches, with the diameter of the longitudinal bore215 being 0.453 inches. The longitudinal bore 215 is 1.02 inches long,with the final approximate 0.09 inches being tapered. The bore 215 maybe drilled using a 29/64 bit with a 118 degree point. The cylindricalmember 200 made of aluminum according to these specifications weighsapproximately 13 grams.

[0027] By way of example, one embodiment of the insert 300 is 1.843inches long, with the flange 320 accounting for 0.100 inches of thislength. The outside diameter of the flange is 0.600 inches. The outsidediameter of the body 310 is 0.500 inches. The longitudinal bore 330 is0.189 inches in diameter, with the enlarged recess 335 being 0.297inches in diameter. The bore 330 may be drilled with a 4.8 mm drillsize, and the recess 335 may be drilled with a 19/64 drill. The insert300 made of brass according to these specifications weighs approximately41 g. Additional inserts shorter in length but of the same dimensions,or made of tungsten or aluminum, also may be utilized for variableweighting. Such weights range from as little as 5 grams for a smallaluminum weight to hundreds of grams. It has been found that weightsabove 250 grams provide only marginal benefit. A typical two-inch, 10-24thread stainless steel socket head cap screw weighs about 9 grams.

[0028] It should be noted that the embodiment of the apparatus of theinvention described above, utilizing the screw 400 in combination withthe bore 330 and small threaded receptacle 226 to secure the weightedinsert to the housing, is only one embodiment of the invention.Alternatively, the threaded receptacle 226 could be of the same diameteras the chamber 222 (i.e., a portion of the walls of chamber 222 would bethreaded to form receptacle 226) with the lower end of the insert 300cooperatively threaded to secure it into the same. Likewise, the upperportion of the walls of chamber 222 could be threaded, with the upperportion of the body of the insert 300 complementarily sized andthreaded, with the body being of a reduced diameter or tapered below thethreads to allow full insertion into the chamber 222. In thisembodiment, the length of the body or angle of taper could be varied toadjust the weight of the insert.

[0029] The method of the present invention modulates the position of thegrip-end nodal location of the fundamental modeshape of a golf club. Thefundamental mode nodal location is a result of the combination of fivefactors: club length, the mass of the club head, the mass of the shaftextension, the mass of the grip, and the mass of the shaft (whichincludes shaft shape and taper, shaft moment of inertia (I), and shaftstiffness (EI)).

[0030] The length of a given golf club affects the distance from thelocation of the grip-end node of the fundamental mode to the end of theclub. This distance is directly proportional to the length of the club.Thus, standard analytical methods used in dimensionless analyses areapplied to simplify comparing clubs of differing lengths. As a result,all length data herein is presented as a percentage of total clublength. For example, if a node is found to be six inches from the gripend of a 36-inch long shaft, the distance will be expressed 16.7% ofshaft length (100 * 6/36=16.7%). The preferred embodiment of theapparatus of the present invention allows variation of the weight of theclub without variation in its length, thus minimizing the effect of onevariable on the dynamic response of the club.

[0031] As noted above, the mass of the club head highly influences thelocation of the head-end node of the fundamental mode, and thefree-pinned boundary condition is utilized for analytical analysis ofthe golf club because the mass of the head drives the fundamental modehead-end node nearly to the end of the entire club. Deviations in themass of the head above approximately 225 grams produce only negligiblechanges in the positions of the fundamental mode grip-end and head-endnode locations.

[0032] Weight appropriately added to the grip end of the club perturbsthe location of the grip-end node and increases the dynamic response ofthe club to the golfer. The empirically measured effect of increasingweights added to the grip end of one golf club on the location of thegrip-end node of the fundamental mode is illustrated in Table 1. This isthe result of grip-end weighting of a Ping Answer II putter without agrip installed.

[0033] As illustrated in Table 1, the location of the grip-end node ofthe fundamental mode was found to be 26.4% of the length of the clubfrom the grip end of the club with no mass added. This value isconsistent with the analytically predicted solution for pinned-freebeams of prismatic shape. A mass of 200 grams added to the grip end ofthe club moved the grip-end node to a point approximately 8% of thelength of the club away from the grip end. This data confirms theeffectiveness of the method of the present invention.

[0034]FIGS. 7A and 7B illustrate graphically examples of the fundamentalmodes of a golf club before (FIG. 7A) and after (FIG. 7B) the locationof the grip-end node has been adjusted in accordance with the presentinvention. The grip-end node is represented by the enlarged dot in thegrip area of the club. Further, based on field testing performed todate, nearly all golfers perceive an improvement in club performancewhen the grip-end node of the fundamental mode is moved upwards closerto the grip end of the shaft and away from the position where thegolfer's hands grip the club. Some golfers prefer a maximum dynamicresponse, which may be achieved by selectively adding weight until theamplitude of the fundamental is maximized in the region of the gripgrasped by the player. Others may prefer more subtle changes in theresponse. The preferred amount of change can be fine tuned to suit suchindividual preferences, as described below.

[0035] The mass of a grip installed on a club influences the magnitudeof movement of the fundamental mode grip-end node position that resultsfrom the addition of the shaft extension mass. The additional mass ofthe extension produces less nodal deviation with the grip installedbecause the grip mass, shaft mass, and extension mass function togetherto define the position of the node. Simply stated, the extension mass isless dominant when the grip is installed. Table 2 illustrates thispoint.

[0036] As with the added mass of the grip, a more massive shaft reducesthe effect of the shaft extension on the position of the grip-end node.It is noteworthy that that standard golf club shafts are not prismaticas they taper from the grip end to the head end. This taper does affectthe head-end node location but it introduces very little perturbation tothe grip-end node location because the taper is generally very small onthe grip end of the club. Nevertheless, to provide a brief explanation,the effect of taper on golf club dynamics results from a change in shaftweight and stiffness. As the shaft tapers, its area moment of inertia(I), a function proportional to the shaft diameter to the fourth power(D4), reduces while the shaft's respective area (A) reduces in relationto the square of the diameter (D2). Bending stiffness (EI) is determinedby the product of modulus of elasticity (E) and the area moment ofinertia (I). Shaft weight is determined by product of the materialdensity (ρ), cross sectional area, and the respective shaft length=ρAL.Thus the stiffness of the shaft reduces faster than the weight.

[0037] Several design parameters thus affect the exact position of thegrip-end node of the fundamental node in response to the added weight.Thus the anticipated perturbation in node location can be bounded toinclude reasonable combinations of the aforementioned design parameters.Fundamental mode grip-end node locations were recorded from a largedatabase of clubs as varying weights were added to the grip-end of theclub, as illustrated in Table 3.

[0038] The results clearly indicate that the change in node position isnonlinearly related to the amount of weight added to the club. Afourth-order polynomial curve fit characterizes these resultsaccurately. According to the data gathered, the addition of weight tothe grip end of the club using the apparatus of the present inventionproduced a minimum fundamental mode grip-end node perturbation describedby the following lower bound equation:

(% Length)=1.45×10⁻¹¹m⁴−1.12×10⁻⁰⁸m³+2.92×10⁻⁰⁶m²−4.22×10⁻⁰⁴m+8.73×10⁻⁰²

[0039] where m equals the mass added to the end of the club. Accordingto the data gathered, the maximum node perturbation is described by thefollowing upper bound equation:

(% Length)=2.35×10⁻¹⁰m⁴−1.52×10⁻⁰⁷m³+3.67×10⁻⁰⁵m²−4.11×10⁻⁰³m+3.28×10⁻⁰¹

[0040] For example, according to the foregoing equations a 100-gramaddition to the grip end of a club will displace the grip-end node aminimum of 6.4% of the club length and a maximum of 15.5% of the length,depending on the mass of the shaft, mass of the club head, and mass ofthe grip installed on the club. For a 34-inch long club, this rangecorrelates to between 2.19 and 5.27 inches from the grip end of theclub. It should be emphasized that the foregoing equations describeupper and lower bounds empirically determined by testing a variety ofclubs.

[0041] For any given club, the mass of the club head, grip, and shaftare fixed and thus the weight added to the grip end can beparametrically varied to displace the grip-end node a desired distancefrom the starting point. This may be accomplished by modal analysis ofthe golf club in a fixture as weight is added, or subjectively by anindividual golfer according to feel.

[0042] Modal analysis of the golf club involves exciting the clubassembly with an electro-dynamic shaker. The golf club is suspended withelastic cords while the shaker is driven with a sinusoidal input. Thefrequency of the input waveform is adjusted until a maximum displacementor amplitude response is observed in the golf club. This frequencyrepresents the golf club's fundamental resonant frequency. With the clubdriven by the shaker at its fundamental resonant frequency, and with anantinode displacement amplitude of approximately 0.5 inch, the grip endnode can be visually identified easily with an accuracy of less than0.05 inch. Weight inserts can then be added to the grip end of the cluband a relationship between the node location and the amount of addedweight can be readily determined. This method can be employed with orwithout the grip installed on the club. This approach is suitable fordetermining and adjusting the location of the grip-end node in a club tobe manufactured, or other relatively large-volume setting. Assuming theend of the club is weighted using the apparatus of the presentinvention, the feel of the club could be further fine tuned by theindividual golfer by adjusting the weight of the insert installed on theshaft extension.

[0043] The method can be practiced for retrofitting individual clubs aswell. Referring to FIGS. 8A-8B, with the grip 510 installed on the shaft500 of a club to be fitted, a small pilot hole 520 is made in the upperend of the grip 510. One of a plurality of variably weighted testweights 600, each with a small pin 610 adapted to mate with the pilothole, is installed on the end of the club. The mass of the sample weightis varied parametrically until the golfer perceives maximum improvementin the dynamic response of the club. In this manner, the initial massmagnitude of the extension 100 is determined so that it can be correctlysized to provide the golfer maximum benefit. For example, some golfersmay prefer a much lighter mass than others, which may call for analuminum cylindrical member 200 with a large diameter longitudinal bore215 in the sleeve 200, while others may prefer a heavier extension,which may call for a brass cylindrical member 200 with a smallerdiameter bore 215. After the cylindrical member 200 has been installedon the shaft, the mass of the inserts 300 can be varied to fine tune thegrip-end node of the fundamental. Further, the golfer can later exchangeinserts to relocate the node in accordance with changes in skill,preference, or course conditions.

[0044] The apparatus of the present invention is advantageous inpracticing the method and therefore is preferred. The further towardsthe grip-end of the club weight is added, the greater its effect uponthe location of the grip-end node. With the apparatus illustrated inFIGS. 1 to 5, the housing 220 and insert 300 comprise the vast majorityof the mass of the extension. They are located above the shaft and atthe end of the club, thus maximizing their effect on the nodal location.Further, the apparatus of the present invention allows the mass of theinsert 300 to be interchanged without varying the length of the club,which enables more precise tuning of the nodal location by varying theweight of the insert only.

[0045] FIGS. 6A-B illustrate an alternative embodiment of the inventionthat is particularly suited for full swing clubs such as drivers. Thisembodiment is similar to the embodiment shown in FIG. 1, but utilizes adifferent securing mechanism and typically has a longer lower sleeve 210and shorter upper housing 220 due to the lesser mass required to improvethe dynamic response of a full swing club. Further, a slightly differentweighted insert 700 is utilized. The inner wall of the longitudinal bore730 is threaded. As shown in FIGS. 6A-6B, a screw 800 is insertedthrough the bore 215 of the cylindrical member 200 and threaded into andthrough the receptacle 226. A portion of the threaded end of the screw800 protrudes into the chamber 222 of the upper housing 220. A set screw810 is threaded into the upper end of the longitudinal bore 730. Anadapter 820, which has a hollow keyed interior, is press fit into theenlarged recess 735 of the bore 730. Preferably, the body 710 of theinsert 700 is of a slightly smaller diameter than the chamber 222, suchthat their respective surfaces are not in contact as described above.The insert 700 is then inserted into the chamber 222 where the lower endof the threaded bore 730 engages the protruding cooperatively threadedend of the screw 800, and the insert 700 is then threaded onto the screw800 and tightened, utilizing adapter 820, until the flange 720 bearsfirmly against the upper end of chamber 222. The set screw 810 isaccessed through the hole in the adapter 820 and tightened firmlyagainst the end of the screw 800 to lock the assembly in place. Notethat the recess 735 could be machined such that the adapter 820 isunnecessary, but the foregoing design allows for decreased manufacturingcosts and the use of inserts from the primary embodiment with a threadedbore. Also, the set screw 810 is optional if a less secure attachment isdesired for a particular application or golfer.

[0046] Although the present invention has been described and shown inconsiderable detail with reference to certain preferred embodimentsthereof, other embodiments are possible. The foregoing description istherefore considered in all respects to be illustrative and notrestrictive. Therefore, the present invention should be defined withreference to the claims and their equivalents, and the spirit and scopeof the claims should not be limited to the description of the preferredembodiments contained herein.

I claim:
 1. A shaft extension for improving the dynamic response of agolf club having an elongated shaft with a lower head end and an uppergrip end, the head end having a club head attached thereto and the gripend being hollow and having an inner and outer diameter, said shaftextension comprising: a cylindrical member comprising a lower sleevesized for snug insertion in said grip end of said shaft and an upperhousing of a diameter slightly larger than the inner diameter of saidgrip end, whereby said housing extends from said shaft when said sleeveis inserted therein, said housing having a chamber therein open at theupper end thereof; an insert of predetermined weight for insertion intosaid housing; means for removably securing said insert to said housing;such that said club may be selectively weighted above the shaft forselectively improving its dynamic response without changing the overalllength of the shaft and shaft extension in combination.
 2. The shaftextension of claim 1 wherein said insert is one of a plurality ofinterchangeable and variably weighted inserts.
 3. The shaft extension ofclaim 1 wherein said insert comprises a body for insertion into saidhousing and further comprises a flange at its upper end having adiameter approximately the same as the outside diameter of the housingwherein the body of said insert may be inserted into said housing untilsaid flange abuts the upper end thereof.
 4. The shaft extension of claim3 wherein said chamber in said housing terminates at its lower end in athreaded receptacle, said one insert comprises a first longitudinal boreaxially aligned with said receptacle, and said securing means comprisesa retaining screw for threaded engagement with said receptacle throughsaid first bore.
 5. The shaft extension of claim 4 wherein the body ofsaid insert is sized such that it is not in contact with the inner wallof said housing, and wherein said retaining screw has a head which ispress fit into a cooperatively sized recess in the upper portion of saidfirst longitudinal bore when said screw is threaded into saidreceptacle.
 6. The shaft extension of claim 4 wherein said lower end ofsaid chamber has a tapered shape of reducing diameter leading into saidthreaded receptacle.
 7. The shaft extension of claim 6 wherein the lowerend of the body of said insert is tapered complementary to said chamber.8. The shaft extension of claim 2 wherein said insert is madeprincipally from a material selected from the group consisting of brass,aluminum, lead, tungsten, titanium, nickel, and stainless steel.
 9. Theshaft extension of claim 2 wherein said sleeve is hollow.
 10. The shaftextension of claim 9 wherein said cylindrical member is made principallyfrom a material selected from the group consisting of brass, aluminum,lead, tungsten, titanium, nickel, and stainless steel.
 11. A shaftextension for improving the dynamic response of a golf club having anelongated shaft with a lower head end and an upper grip end, the headend having a club head attached thereto and the grip end being hollowand having an inner and outer diameter, said shaft extension comprising:a cylindrical member comprising a lower sleeve of a diameter slightlysmaller than the inner diameter of said grip end of said shaft for snuginsertion therein and an upper housing of a diameter slightly largerthan the inner diameter of said grip end of said shaft, whereby saidhousing extends from said shaft when said sleeve is inserted therein,said housing having an interior chamber with an opening at the top ofsaid housing; an insert of predetermined weight removably secured withinsaid housing; such that said club may be selectively weighted above theshaft for selectively improving its dynamic response, without changingthe overall length of the shaft and shaft extension in combination. 12.The shaft extension of claim 11, wherein said insert is one of aplurality of interchangeable and variably weighted inserts.
 13. Theshaft extension of claim 11 wherein said insert comprises a body forinsertion into said housing and further comprises a flange at its upperend having a diameter approximately the same as the outside diameter ofthe housing wherein the body of said insert may be inserted into saidhousing until said flange abuts the upper end thereof.
 14. The shaftextension of claim 13 wherein said insert is removably secured withinsaid chamber with a fastener.
 15. The shaft extension of claim 14wherein said chamber terminates at its lower end in a threadedreceptacle, said insert has a first longitudinal bore coaxial with saidthreaded receptacle, and said fastener is a retaining screw, such thatsaid retaining screw may be threaded into said receptacle through saidsecond bore.
 16. The shaft extension of claim 15 wherein the body ofsaid insert is sized such that it is not in contact with the inner wallof said housing, and wherein said retaining screw has a head which ispress fit into a cooperatively sized recess in the upper portion of saidfirst longitudinal bore when said screw is threaded into saidreceptacle.
 17. The shaft extension of claim 16 wherein said lower endof said chamber has a tapered shape of reducing diameter leading intosaid threaded receptacle.
 18. The shaft extension of claim 17 whereinthe lower end of the body of said insert is tapered complementary tosaid chamber.
 19. The shaft extension of claim 12 wherein said insert ismade principally from a material selected from the group consisting ofbrass, aluminum, lead, tungsten, titanium, nickel, and stainless steel.20. The shaft extension of claim 12 wherein said sleeve is hollow. 21.The shaft extension of claim 20 wherein said cylindrical member is madeprincipally from a material selected from the group consisting of brass,aluminum, lead, tungsten, titanium, nickel, and stainless steel.
 22. Amethod of improving the dynamic response of a golf club, said clubhaving an elongated shaft with a lower head end and an upper grip end,the head end having a club head attached thereto, and wherein the golfclub in response to a predetermined input exhibits a fundamentalmodeshape with a grip-end node at a first location, said methodcomprising selectively adding weight to the grip end of said club untilsaid grip-end node moves from said first position to a second positioncloser to said grip end.
 23. The method of claim 22 wherein said shafthas a gripping region below said grip end which is grasped by a golferduring play and wherein said weight is selectively added to said gripend of said club until the amplitude of the fundamental mode ismaximized within said gripping region.
 24. The method of claim 22wherein said weight is selectively added to said club using the deviceof claim
 1. 25. The method of claim 22 wherein said weight isselectively added to said club using the device of claim 11.